^CryStalline SiliC°n °arbide is Produced «* about 1,840°C. and dissociates at about 40 C. Fusion or softening does not occur below this point. Under actively oxidizing conditions decomposition undoubtedly occurs to a certain extent. But oxidation goes on slowly when in contact with gases containing but a small amount of tree oxygen. For many purposes these refractories are exceedingly useful owing to their high thermal conductivity, their low coefficient of expansion and their rigidity and mechanical strength. The specific heat is given as being 0.162 for the material bonded by the cementation process and 0.180 for the one bonded with clay. The thermal conductivity of the former is 0.0275 and for the latter 0.0243. The conductivity of the silicon carbide refractories is therefore about seven times that of clay firebrick. The coefficient of thermal expansion is 0.0000047 per degree Centigrade, which is lower than that of clay materials. The porosity of silicon carbide products varies^ between 15 to 20 per cent and the bulk specific gravity from 2.1 to 2.5. The material resists load conditions at furnace temperatures (1,400 to 1,450°C.) without any appreciable deformation.
Special Refractories.—Under certain conditions refractories of special qualities may be employed such as zirconium oxide, zirconium silicate, chromite, fused silica, boron nitride, aluminum nitride, lime, beryllium oxide, cerium dioxide, thoria, asbestos and various synthetic combinations.
Zirconia.—The sources of this oxide are the silicate, ZrSi04, zircon, baddeley-ite, containing on an average 84 per cent of Zr02, and the monazite sands. Pure zirconia fuses at about 2,600°C. which is depressed by the presence of silica, iron, etc. The zirconia must first be calcined at as high a temperature as possible, up to 2,000°C., according to the amount of impurities present, then ground'and molded either by pressure, by hand molding or by casting. In the first two processes an organic binder like linseed oil, starch, etc., may be employed; in the latter water is used and the procedure is like that of ceramic casting. From the crude zirconia or zircon, iron and some silica may be eliminated by treatment with chlorine at about 800° C. The thermal conductivity of Zr02 is quite low and the coefficient of expansion approaches that of quartz glass, being about 0.00000084. It resists fused alkalies very satisfactorily. Pure zirconia has a specific electrical conductivity of 0.0008 reciprocal ohms at 1,200°C. and 0.0034 at 1,400°. The addition of alumina increases the conductivity. Thus one molecule of AloOs added to nine of zirconia increases the conductivity at 1,200°C. to 0.00255 reciprocal ohms. Cerium oxide, used in the same proportion, gives a conductivity of 0.0075 reciprocal ohms at 1,000° and ferric oxide 0.0358 at 1,287°. Zirconia refractories therefore conduct the electric current when heated. Thus the pencils of the Nernst lamp containing about 85 per cent of zirconia and 15 per cent yttria become conductors, a principle which has also been employed in the construction of electric furnaces.
' Through fine grinding in water the zirconia or zirconium silicate may be reduced to a state where it may be molded without the addition of cementing materials. As in the case of all highly refractory oxides the addition of such silicates as clay should be avoided owing to the great depression of the melting point. The contraction of zirconia in calcination may be accelerated through the use of a small amount of boric
Zirconia combines with carbon to form a carbide which is also an abrasive. Combining zirconia with small quantities of silica, beryllia, magnesia and alumina lowers the melting point (alumina being the least detrimental), while thoria and yttria raise